Sunday, December 21, 2008

Nature versus naturoid

[This is my Materials Witness column for the January 2009 issue of Nature Materials.]

Are there metameric devices in the same way that there are metameric colours? The latter are colours that look identical to the eye but have different spectra. Might we make devices that, while made up of different components, perform identically?

Of course we can, you might say. A vacuum tube performs the same function as a semiconductor diode. Clocks can be driven by springs or batteries. But the answer may depend on how much similarity you want. Semiconductor diodes will survive a fall on a hard floor. Battery-operated clocks don’t need winding. And what about something considerably more ambitious, such as an artificial heart?

These thoughts are prompted by a recent article by sociologist Massimo Negrotti of the University of Urbino in Italy (Design Issues24(4), 26-36; 2008). Negrotti has for several years pondered the question of what, in science and engineering, is commonly called biomimesis, trying to develop a general framework for what this entails and what its limitations might be. His vision is informed less by the usual engineering concern, evident in materials science, to learn from nature and imitate its clever solutions to design problems; rather, Negrotti wants to develop something akin to a philosophy of the artificial, analogous to (but different from) that expounded by Herbert Simon in his 1969 book The Sciences of the Artificial.

To this end, Negrotti has coined the term ‘naturoid’ to describe “all devices that are designed with natural objects in mind, by means of materials and building procedures that differ from those that nature adopts.” A naturoid could by a robot, but also a synthetic-polymer-based enzyme, an artificial-intelligence program, even a simulant of a natural odour. This concept was explored in Negrotti’s 2002 book Naturoids: On the Nature of the Artificial (World Scientific, New Jersey).

Can one say anything useful about a category so broad? That might remain a matter of taste. But Negrotti’s systematic analysis of the issues has the virtue of stripping away some of the illusions and myths that attach to attempts to ‘copy nature’.

It won’t surprise anyone that these attempts will always fall short of perfect mimicry; indeed that is often explicitly not intended. Biomimetic materials are generally imitating just one function of a biological material or structure, such as adhesion or toughness. Negrotti calls this the ‘essential performance’, which itself implies also a selected ‘observation level’ – we might make the comparison solely at the level of bulk mechanical behaviour, irrespective of, say, microstructure or chemical composition.

This inevitably means that the mimicry breaks down at some other observation level, just as colour metamerism can fail depending on the observing conditions (daylight or artificial illumination, say, or different viewing angles).

This reasoning leads Negrotti to conclude that there is no reason to suppose the capacities of naturoids can ever converge on those of the natural models. In particular, the idea that robots and computers will become ever more humanoid in features and function, forecast by some prophets of AI, has no scientific foundation.

Physicists’ understandable embarrassment that we don’t know what most of the universe is made of prompts an eagerness, verging on desperation, to identify the missing ingredients. Dark energy – the stuff apparently causing an acceleration of cosmic expansion – is currently a matter of mere speculation, but dark matter, which is thought to comprise around 85 percent of tangible material, is very much on the experimental agenda. This invisible substance is inferred on several grounds, especially that galaxies ought to fall apart without its gravitational influence. The favourite idea is that dark matter consists of unknown fundamental particles that barely interact with visible matter – hence its elusiveness.

One candidate is a particle predicted by theories that invoke extra dimensions of spacetime (beyond the familiar four). So there was much excitement at the recent suggestion that the signature of these particles has been detected in cosmic rays, which are electrically charged particles (mostly protons and electrons) that whiz through all of space. Cosmic rays can be detected when they collide with atoms in the Earth’s atmosphere. Some are probably produced in high-energy astrophysical environments such as supernovae and neutron stars, but their origins are poorly understood.

An international experiment called ATIC, which floats balloon-borne cosmic-ray detectors high over Antarctica, has found an unexpected excess of cosmic-ray electrons with high energies, which might be the debris of collisions between the hypothetical dark-matter particles. That’s the sexy interpretation. They might instead come from more conventional sources, although it’s not then clear whence this excess above the normal cosmic-ray background.

The matter is further complicated by an independent finding, from a detector called Milagro near Los Alamos in New Mexico, that high-energy cosmic-ray protons seem to be concentrated in a couple of bright patches in the sky. It’s not clear if the two results are related, but if the ATIC electrons come from the same source as the Milagro protons, that rules out dark matter, which is expected to produce no such patchiness. On the other hand, no other source is expected to do so either. It’s all very perplexing, but nonetheless a demonstration that cosmic rays, whose energies can exceed those of equivalent particles in Cern’s new Large Hadron Collider, offer an unparalleled natural resource for particle physicists.

***** A Californian biotech company is promising, within five years, to be able to sequence your entire personal genome while you wait. In under an hour, a doctor could deduce from a swab or blood sample all of your genetic predispositions to disease. At least, that’s the theory.

Pacific Biosciences in Menlo Park has developed a technique for replicating a piece of DNA in a form that contains fluorescent chemical markers attached to each ‘base’, the fundamental building blocks of genes. Each of the four types of base gets a differently coloured marker, and so the DNA sequence – the arrangement of bases along the strand – can be discerned as a string of fairy lights, using a microchip-based light sensor that can image individual molecules.

With a readout rate of about 4.7 bases per second, the method would currently take much longer than an hour to sequence all three billion bases of a human genome. And it is plagued by errors – mistakes about the ‘colour’ of the fluorescent markers – which might wrongly identify as many as one in five of the bases. But these are early days; the basic technology evidently works. The company hopes to start selling commercial products by 2010.

Faster genome sequencing should do wonders for our fundamental understanding of, say, the relationships between species and how these have evolved, or the role of genetic diversity in human populations. There’s no doubt that it would be valuable in medicine too – for example, potential drugs that are currently unusable because of genetically based side-effects in a minority of cases could be rescued by screening that identifies those at risk. But many researchers admit that the notion of a genome-centred ‘personalized medicine’ is easily over-hyped. Not all diseases have a genetic component, and those that do may involve complex, poorly understood interactions of many genes. Worse still, DIY sequencing kits could saddle people with genetic data that they don’t know how to interpret or deal with, as well as running into a legal morass about privacy and disclosure. At this rate, the technology is far ahead of the ethics.

***** Besides, it is becoming increasingly clear that the programme encoded in genes can be over-ridden: to put it crudely, an organism can ‘disobey’ its genes. There are now many examples of ‘epigenetic’ inheritance, in which phenotypic characteristics (hair colour, say, or susceptibility to certain diseases) can be manifested or suppressed despite a genetic imperative to the contrary (see Prospect May 2008). Commonly, epigenetic inheritance is induced by small strands of RNA, the intermediary between genes and the proteins they encode, which are acquired directly from a parent and can modify the effect of genes in the offspring.

An American team have now shown a new type of such behaviour, in which a rogue gene than can cause sterility in crossbreeds of wild and laboratory-bed fruit flies may be silenced by RNA molecules if the gene is maternally inherited, maintaining fertility in the offspring despite a ‘genetic’ sterility. Most strikingly, this effect may depend on the conditions in which the mothers are reared: warmth boosts the fertility of progeny. It’s not exactly inheritance of acquired characteristics, but is a reminder, amidst the impending Darwin celebrations, of how complicated the story of heredity has now become.

Monday, December 08, 2008

Who knows what ET is thinking?

[My early New Year resolution is to stop giving my Nature colleagues a hard time by forcing them to edit stories that are twice as long as they should be. It won’t stop me writing them that way (so that I can stick them up here), but at least I should do the surgery myself. Here is the initial version of my latest Muse column, before it was given a much-needed shave.]

Attempts to identify the signs of astro-engineering by advanced civilizations aren’t exactly scientific. But it would be sad to rule them out on that score.

“Where is everybody?” Fermi’s famous question about intelligent extraterrestrials still taunts us. Even if the appearance of intelligent life is rare, the vast numbers of Sun-like stars in the Milky Way alone should compensate overwhelmingly, and make it a near certainty that we are not alone. So why does it look that way?

Everyone likes a good Fermi story, but it seems that the origins of the ‘Fermi Paradox’ are true [1]. In the summer of 1950, Fermi was walking to lunch at Los Alamos with Edward Teller, Emil Konopinski and Herbert York. They were discussing a recent spate of UFO reports, and Konopinski recalled a cartoon he had seen in the New Yorker blaming the disappearance of garbage bins from the streets of New York City on extraterrestrials. And so the group fell to debating the feasibility of faster-than-light travel (which Fermi considered quite likely to be found soon). Then they sat down to lunch and spoke of other things.

Suddenly, Fermi piped up, out of the blue, with his question. Everyone knew what he meant, and they laughed. Fermi apparently then did a back-of-the-envelope calculation (his forte) to show that we should have been visited by aliens long ago. Since we haven’t been (nobody mention Erich von Daniken, please), this must mean either that interstellar travel is impossible, or deemed not worthwhile, or that technological civilizations don’t last long.

Fermi’s thinking was formalized and fleshed out in the 1960s by astronomer Frank Drake of Cornell University, whose celebrated equation estimates the probability of extraterrestrial technological civilizations in our galaxy by breaking it down into the product of the various factors involved: the fraction of habitable planets, the number of them on which life appears, and so on.

Meanwhile, the question of extraterrestrial visits was broadened into the problem of whether we can see signs of technological civilizations from afar, for example via radio broadcasts of the sort that are currently sought by the SETI Project, based in Mountain View, California. This raises the issue of whether we would know signs of intelligence if we saw them. The usual assumption is that a civilization aiming to communicate would broadcast some distinctive universal pattern such as an encoding of the mathematical constant pi.

A new angle on that issue is now provided in a preprint [2] by physicist Richard Carrigan of (appropriately enough) the Fermi National Accelerator Laboratory in Batavia, Illinois. He has combed through the data from 250,000 astronomical sources found by the IRAS infrared satellite – which scanned 96 percent of the sky – to look for the signature of solar systems that have been technologically manipulated after a fashion proposed in the 1960s by physicist Freeman Dyson.

Dyson suggested that a sufficiently advanced civilization would baulk at the prospect of its star’s energy being mostly radiated uselessly into space. They could capture it, he said, by breaking up other planets in the solar system into rubble that formed a spherical shell around the star, creating a surface on which the solar energy could be harvested [3].

Can we see a Dyson Sphere from outside? It would be warm, re-radiating some of the star’s energy at a much lower temperature – for a shell with a radius of the Earth’s orbit around a Sun-like star, the temperature should be around 300 K. This would show up as a far-infrared object unlike any other currently known. If Dyson spheres exist in our galaxy, said Dyson, we should be able to see them – and he proposed that we look.

That’s what Carrigan has done. He reported a preliminary search in 2004 [4], but the new data set is sufficient to spot any Dyson Spheres around sun-like bodies out to 300 parsecs – a volume that encompasses a million such stars. It will probably surprise no one that Carrigan finds no compelling candidates. One complication is that some types of star that might resemble a Dyson Sphere, such as those in the late stage of their evolution when they become surrounded by thick dust clouds. But there are ways to weed these out, for example by looking at the spectral signatures such objects are expected to exhibit. Winnowing out such false positives left just 17 candidate objects, of which most, indeed perhaps all, could be given more conventional interpretations. It’s not quite the same as saying that the results are wholly negative – Carrigan argues that the handful of remaining candidates warrant closer inspection – but there’s currently no reason to suppose that there are indeed Dyson Spheres out there.

Dyson says that he didn’t imagine in 1960 that a search like this would be complicated by so many natural mimics of Dyson Spheres. “I had no idea that the sky would be crawling with millions of natural infrared sources”, he says. “So a search for artificial sources seemed reasonable. But after IRAS scanned the sky and found a huge number of natural sources, a search for artificial sources based on infrared data alone was obviously hopeless.”

All the same, he feels that Carrigan may be rather too stringent in whittling down the list of candidates. Carrigan basically excludes any source that doesn’t radiate energy pretty much like a ‘black body’. “I see no reason to expect that an artificial source should have a Planck [black-body] spectrum”, says Dyson. “The spectrum will depend on many unpredictable factors, such as the paint on the outside of the radiating surface.”

So although he agrees that there is no evidence that any of the IRAS sources is artificial, he says that “I do not agree that there is evidence that all of them are natural. There are many IRAS sources for which there is no evidence either way.”

Yet the obvious question hanging over all of this is: who says advanced extraterrestrials will want to make Dyson Spheres anyway? Dyson’s proposal carries a raft of assumptions about the energy requirements and sources of such a civilization. It seems an enormously hubristic assumption that we can second-guess what beings considerably more technologically advanced than us will choose to do (which, in fairness, was never Dyson’s aim). After all, history shows that we find it hard enough to predict where technology will take us in just a hundred years’ time.

Carrigan concedes that it’s a long shot: “It is hard to predict anything about some other civilization”. But he says that the attraction of looking for the Dyson Sphere signature is that “it is a fairly clean case of an astroengineering project that could be observable.”

Yet the fact is that we know absolutely nothing about civilizations more technologically advanced than ours. In that sense, while it might be fun to speculate about what is physically possible, one might charge that this strays beyond science. The Drake equation has itself been criticized as being unfalsifiable, even a ‘religion’ according to Michael Crichton, the late science-fiction writer.

All that is an old debate. But it might be more accurate to say that what we really have here is an attempt to extract knowledge from ignorance: to apply the trappings of science, such as equations and data sets, to an arena where there is nothing to build on.

There are, however, some conceptual – one might say philosophical – underpinnings to the argument. By assuming that human reasoning and agendas can be extrapolated to extraterrestrials, Dyson was in a sense leaning on the Copernican principle, which assumes that the human situation is representative rather than extraordinary. It has recently been proposed [5,6] that this principle may be put to the experimental test in a different context, to examine whether our cosmic neighbourhood is or is not unusual – whether we are, say, at the centre of a large void, which might provide a prosaic, ‘local’ explanation for the apparent cosmic acceleration that motivates the idea of dark energy.

But the Copernican principle can be considered to have a broader application than merely the geographical. Astrophysicist George Ellis has pointed out how arguments over the apparent fine-tuning of the universe – the fact, for example, that ratio of the observed to the theoretical ‘vacuum energy’ is the absurdly small 10**-120 rather than the more understandable zero – entails an assumption that our universe should not be ‘extraordinary’. With a sample of one, says Ellis, there is no logical justification for that belief: ‘there simply is no proof the universe is probable’ [7]. He argues that cosmological theories that use the fine-tuning as justification are therefore drawing on philosophical rather than scientific arguments.

It would be wrong to imagine that a question lies beyond the grasp of science just because it seems very remote and difficult – we now have well-motivated accounts of the origins of the moon, the solar system, and the universe itself from just a fraction of a second onward. But when contingency is involved – in the origin of life, say, or some aspects of evolution, or predictions of the future – the dangers of trying to do science in the absence of discriminating evidence are real. It becomes a little like trying to figure out the language of Neanderthals, or the thoughts of Moses.

It is hard to see that a survey like Carrigan’s could ever claim definitive, or even persuasive, proof of a Dyson Sphere; in that sense, the hypothesis that the paper probes might indeed be called ‘unscientific’ in a Popperian sense. And in the end, the Fermi Paradox that motivates it is not a scientific proposition either, because we know precisely nothing about the motives of other civilizations. Astronomer Glen David Brin suggested in 1983, for example, that they might opt to stay hidden from less advanced worlds, like adults speaking softly in a nursery ‘lest they disturb the infant’s extravagant and colourful time of dreaming’ [8]. We simply don’t know if there is a paradox at all.

But how sad it would be to declare out of scientific bounds speculations like Dyson’s, or experimental searches like Carrigan’s. So long as we see them for what they are, efforts to gain a foothold on metaphysical questions are surely a valid part of the playful creativity of the sciences.

For any of you who reads Italian (I am sure there are many), there is a little essay of mine up on the Italian science & culture site Fortepiano here. This is basically the text of the short talk I gave in Turin for the receipt of one of the Lagrange prizes for complexity last April. At least, I hope it is – my Italian is non-existent, I fear. Which is a shame, because the Fortepiano site looks kind of intriguing.